A TN liquid-crystal cell is usually formed from two transparent substrates joined together to form between them a cavity into which the liquid-crystal molecules are injected. A liquid-crystal molecule has a shape elongate along a longitudinal axis, such as a rod. The pretilt angle can then be defined as follows: a normal position of such a molecule, flat against the surface plane of a substrate, is considered; one end of the molecule is anchored on this plane; and the other end is forced into a position such that the longitudinal axis of the molecule makes an angle with the surface plane of the substrate—this is the pretilt angle.
This angle notably makes it possible to impose the helical rotation direction on the liquid-crystal molecules of the cell through the thickness of the cavity and has a direct impact on the performance of a liquid-crystal cell. It is obtained in a well-known manner by depositing, on each of the faces of the substrates internal to the cavity, a transparent alignment layer, typically a polyimide layer, which is for example rubbed by means of a fabric roller along a defined orientation axis, so as to create anchorage lines on the surface along this axis and which is subjected to various cleaning and thermal annealing treatments. The rubbing operations carried out on the two substrates and their adjoining are such that a twist angle exists between the rubbing axes of the two alignment layers. For example, this twist angle is 90° or π/2 rad. Maybe lower, for example 80°, or higher, for example 280°, typically for what are called STN (super twisted nematic) liquid crystals. Hereafter, the expression “TN liquid crystal” is understood for any twist angle.
The liquid-crystal molecules all have the defined angle of inclination at rest, called the pretilt angle, relative to the plane of the substrate. When an electric field is applied, a helical rotation direction of the liquid-crystal molecules is imposed through the thickness of the cavity. The rubbing axes of the two alignment layers in the assembled cell define the top position and bottom position of the cell in a display, these being notions associated with the contrast values at the viewing angle.
The effective pretilt angle obtained at the end of manufacture depends on various factors, a number of which may be mentioned here: the alignment layer rubbing conditions; the cleaning steps; the topography of each of the substrates; the material or materials (polyimides) used to produce the alignment layers; the properties of the liquid crystals injected; etc.
Notably, the topographies of the two substrates are different. Typically, taking for example a liquid-crystal display comprising an active-matrix cell and color filters, a first substrate corresponds to the active matrix, which notably comprises TFT transistors, select lines, data lines and the first pixel electrodes of the screen, and a second substrate corresponds to the back electrode forming the other pixel electrode common to all the pixels, and also comprises red, green and blue color filters. The array of filters on the substrate 2 and the TFT array with the pixel electrodes on the substrate 1 give very different substrate topographies. Owing to these different topographies of the two substrates, the pretilt angle of the cell depends on that point on the surface where it is observed. In practice, the pretilt angle of a cell is an average value. This is because the value of this angle may be different on one substrate from that on the other. It may vary along the three dimensions, that is to say, considering a molecule, the angle may vary on the position of this molecule in the plane of the substrates and through the thickness of the cavity. If any step or steps in the manufacturing process are defective, it will be understood that this will have a direct impact on the pretilt angle of the cell and consequently on the quality of the display.
In the invention, if the display defects are observed at the end of manufacture during the test and measurement phase on the cell, for example if streaking, light leakage or angle contrast defects are observed, it will be desirable to check the average value of the pretilt angle of the cell in order to rapidly identify, as the case may be, one or more of the manufacturing process phases responsible for the observed defect or defects: contamination, obsolescence or defects in the polyimide, error in the rubbing operation.
Usually, the pretilt angle measurement is used in the procedure for developing new manufacturing processes or processes using a new material. Within this development context, the measurement is carried out by means of a well-known method, called the rotating-crystal method described by T. J. Scheffer and J. Nehring. This method uses specially manufactured cells. These cells may be immersed in liquid of matched index so as to eliminate Fresnel reflections, thereby enabling the precision of the measurement to be improved.
This measurement method is very suitable for cells that are specially designed for it, which are simplistic assemblies adapted to the parameter that it is desired to characterize. However, it cannot be used to test defective liquid-crystal screens at the end of manufacture. It is not suitable for measuring a pretilt angle in a cell but is a true product finished in all its complexity on leaving the production line.
In the invention, a solution to this technical problem is proposed by means of a method that does not require the production of special cells and can be readily used directly on liquid-crystal cell production lines.
According to the invention, an average value of the pretilt angle of a liquid-crystal cell is determined, notably by comparing a transmission measurement at an angle in a plane defined by the tested cell with a calculated theoretical value. More precisely, to determine the tilt angle in a twisted nematic liquid-crystal cell, the transmission of the cell is calculated as a function of the angle of incidence α for a light beam of given wavelength λ using apparent values of the thickness d of the cell cavity, of the extraordinary index ne and of the twist angle θtwist, in order to obtain a plurality of simulation curves, one per pretilt angle. The transmission curve is measured as a function of the angle of incidence of the cell along the YY′ axis passing through the top T and bottom B positions of the cell, for the light beam of wavelength λ, using a contrast meter. The coincidence of this measurement curve with one of the plurality of simulation curves gives the pretilt angle of this cell.
The invention therefore relates to a method of determining a pretilt angle in a liquid-crystal cell, said cell comprising a cavity between two substrates containing liquid-crystal molecules, each face of the substrates inside the cavity comprising an alignment layer such that the liquid-crystal molecules are each inclined to the plane of the substrates by said pretilt angle, said liquid crystal being of the twisted nematic type with a defined twist angle, and said cell having a top position and a bottom position which define a vertical mid-axis when viewed from the front. The method includes use of a contrast meter for establishing a measurement curve, along said vertical axis of the cell, for the transmission of a light beam at a defined wavelength of said cell as a function of the angle of incidence of a light beam on said cell, and a comparison of said measurement curve with a plurality of simulation curves for the transmission of said cell as a function of the angle of incidence of the light beam along said vertical axis of the cell, each of said plurality of simulation curves being calculated for a defined pretilt value and said plurality of curves being calculated by taking apparent values of the thickness d of the liquid-crystal cavity between the two substrates, of the extraordinary index ne of the liquid crystals and of the twist angle of the cell which are determined by the following formulae:
                    d        app            =              d                  cos          ⁡                      (            γ            )                                ;                      ne        app            =              ne                              1            +                                          (                                                                            ne                      2                                                              no                      2                                                        -                  1                                )                            ⁢                              sin                ⁡                                  (                                      γ                    -                                          θ                      tilt                                                        )                                                                          ;              θ              twist        -        a              =          2      ·              Arctan        (                  1                      cos            ⁡                          (              γ              )                                      )            where γ is the transmission angle of the light beam in the liquid crystal, which depends on the angle of incidence, no is the ordinary index of the liquid crystals, d is the thickness of the cavity and ne is the extraordinary index at normal incidence.
The determination of the pretilt angle of said cell is given by selecting a simulation curve from said plurality of curves that coincide substantially with said measurement curve.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious aspects, all without departing from the invention.
Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive.